1. Field of the Invention
The present invention relates to a combined electron microscope capable of displaying SEM/STEM images, and a TEM image and, more particularly, to an electron microscope permitting one to search for a field of view containing features of interest in a shorter time.
2. Description of the Related Art
Conventionally, combined TEM/STEM/SEM instruments have been developed as attachments to TEM instruments. In recent years, analytical functions offered by elemental analysis using X-ray spectroscopy or by electron energy loss spectroscopy have tended to be added in use. Point analysis, line analysis, or area analysis are performed on a specimen. That is, information is obtained from the specimen by local elemental analysis using the SEM/STEM mode.
FIG. 8 conceptually illustrates the configuration in the SEM/STEM mode. An electron beam focused by an electron lens L1 is deflected and scanned across a specimen 1 by a deflector 8. At this time, secondary electrons emanating from the surface of the specimen 1 are detected by a detector 2. For example, the detector 2 consists of a combination of a scintillator and a photomultiplier tube. When electrons strike the scintillator, light is emitted. The photomultiplier tube receives the emitted light and reconverts it into electrons, thus multiplying electrons.
A secondary electron signal obtained by the detector 2 is amplified by a following amplifier 3, and is sent as a brightness signal to a CRT 4. Thus, the brightness of the CRT 4 is varied according to the magnitude of the secondary electrons ejected by the specimen 1. That is, an SEM image is displayed on the viewing screen of the CRT 4. The mode of operation described thus far is referred to as the SEM mode or the SEM imaging mode.
Meanwhile, the electron beam passed through the specimen 1 is focused by an electron lens L2 and made to enter a detector 5, which in turn produces an electrical signal corresponding to the impinging, transmitted electron beam. For instance, this detector 5 can be a combination of a scintillator and a photomultiplier tube as mentioned previously. The output from this detector 5 is fed to a following amplifier 6, where the signal is amplified. The amplified transmission electron signal varies the brightness on another CRT 7 according to the magnitude of the electron beam transmitted through the specimen 1. As a result, an STEM image is displayed on the CRT 7. The mode of operation described thus far is referred to as the STEM mode or the STEM imaging mode. Note that a single CRT can be used as both CRTs 4 and 7.
A scintillator/photomultiplier combination is used for each of the SEM detector 2 and the STEM detector 5. However, the SEM detector 2 is provided with an electrode for pulling in electrons, unlike the STEM detector 5. A voltage of about +10 kV is applied to this electrode.
FIG. 9 conceptually illustrates the configuration in the TEM mode. The electron beam EB passed through an electron lens L3 is directed onto the specimen 1. At this time, the electron beam passed through the specimen 1 is enlarged and projected onto a fluorescent screen 10 by an electron lens L4. The operator can observe the image on the fluorescent screen.
The TEM image displayed on the fluorescent screen 10 is focused onto a TV camera 12 by an optical lens and/or optical fiber 11. The image is taken by the TV camera 12 and displayed on a CRT 13. The mode of operation described thus far is referred to as the TEM mode or the TEM imaging mode. In the case of a combined SEM/STEM/TEM instrument, the CRT 13 may be common with the CRT 4 or 7. The difference between SEM/STEM image(s) and the TEM image is as follows. In the SEM/STEM mode, a sharply focused electron beam is scanned across a specimen. In the TEM mode, a specimen is illuminated by a parallel beam of a large diameter.
Usually, the user searches for a field of view containing distinctive morphological features, such as crystal defects or crystal grain boundaries, by TEM and analyzes the field of view. Sometimes, the user may search for a field of view of interest (i.e., containing features) in the SEM/STEM mode and obtain a high-resolution image or electron diffraction pattern at that location in the TEM mode.
Where the combined instrument is used for the purpose described above, the correspondence between the TEM image and the SEM/STEM image(s) is very important. If their fields of view differ greatly, the user is prompted to search for a desired field of view in each mode of operation. To solve this problem, a memory for storing the values of electric current through the electron beam deflector is provided for each of the TEM mode and the SEM/STEM mode. In this way, the fields of view in the two modes are brought into coincidence.
However, the SEM/STEM images and the TEM image differ in nature. In particular, in an SEM image, areas where particles are present are brighter. In an STEM or TEM image, areas where particles are present are darker. In this manner, it is difficult to make their fields of view correspond to each other.
Furthermore, in the conventional electron microscope, a different detection method is used in each different mode of operation. In the TEM mode, the image rotates while imaging is being performed. Therefore, it has been difficult to ascertain the field of view in each mode. Furthermore, in the conventional microscope, a TEM image is displayed on the fluorescent screen in the image observation chamber, while SEM/STEM images are displayed on a CRT. This makes it difficult to compare the images created in the two different modes.
It is an object of the present invention to provide a combined electron microscope in which the SEM/STEM images and the TEM image can be made to precisely correspond to each other.
A first embodiment of the present invention for solving the foregoing problems is a combined electron microscope having an SEM/STEM mode and a TEM mode and capable of producing observable SEM/STEM images and TEM images. The microscope comprises a scanning device for the SEM/STEM mode, a storage memory in which magnifications of TEM images and angles of rotation of the images at those magnifications are stored, a computer control system for making corrections based on the data stored in the storage memory to match the SEM/STEM images in the SEM/STEM mode to the angles of rotation of the TEM images by controlling the scanning device, and a display device for displaying the images corrected by the computer control system.
Because of this structure, the angle of rotation (orientation) of the displayed SEM/STEM images can be matched to the displayed TEM image that rotates according to the magnification. Consequently, corresponding areas of the SEM/STEM images and TEM image can be displayed on the same display device at matched magnifications and at matched angles of rotation. Hence, the SEM/STEM images and TEM image can be made to correspond to each other precisely.
A second embodiment of the invention is a combined electron microscope having an SEM/STEM mode and a TEM mode and capable of producing observable SEM/STEM images and TEM images. The microscope comprises scanning means for the SEM/STEM mode, plural imaging lenses for the TEM mode, a first memory in which magnifications of the TEM images relative to angles of rotation of the SEM/STEM images and values of electrical current supplied to the imaging lenses for the TEM mode to match the angles of rotation of TEM images to the angles of rotation of SEM/STEM images are stored, a second memory in which angles of rotation of TEM images relative to the magnifications of the TEM images to match the angles of rotation of SEM/STEM images to the angles of rotation of the TEM images are stored, a computer for correcting angles of rotation based on data stored in the first or second memory and according to the used imaging mode, and a display device for displaying the image or images corrected by the computer control system.
In this structure, the orientation of a displayed TEM image can be aligned to SEM/STEM images that are rotated arbitrarily and displayed. Furthermore, the angle of rotation (orientation) of displayed SEM/STEM images can be aligned to a displayed TEM image rotating according to the magnification. In consequence, corresponding areas of the SEM/STEM and the TEM images can be displayed on the same display device at matched magnifications and at matched angles of rotation. Hence, the SEM/STEM and the TEM images can be made to correspond to each other precisely.
Other objects and features of the invention will appear in the course of the description thereof, which follows.